1. Field of the Invention
The invention relates to demodulating signals which convey binary data by angle modulation of a carrier, and it relates more particularly to a demodulator for constant envelope and continuous phase signals which have been angle modulated by a train of binary symbols.
Constant envelope modulation schemes are widely used at present in radio communications because of their immunity to non-linear amplitude distortion. Of such modulation schemes, those in which the phase variation takes place exclusively during the transmission period for one bit (total response modulation schemes) occupy a wide spectrum and thus limit the number of channels that can be transmitted over a given bandwidth. Conversely, continuous phase modulation in which the phase variation due to any one bit is spread over a plurality of bit periods (partial response modulation) has advantageous properties of considerably reduced spectrum occupation.
2. Description of the Prior Art
Demodulation is conventionally performed either by means of a coherent demodulator having a reference phase, or else by means of a non-coherent demodulator which may be of the differential type.
At any given instant, a coherent demodulator gives a value representative of the frequency or of the phase of a signal relative to the corresponding quantity of a reference oscillator. To do this, a coherent demodulator is associated with a phase-lock loop which requires finite settling time. This can be incompatible with some types of application, e.g. in frequency hopping links. In such applications, it is difficult to obtain a satisfactory reference phase at all instants because the random phase rotation in the transmission channel varies with frequency, i.e. at each frequency hop.
Further, non-coherent demodulation of the differential type is not always suitable for demodulating signals in which the phase variation is spread over a plurality of bits because of intersymbol interference.
Preferred embodiments of the invention provide a non-coherent type demodulator, i.e. a demodulator which does not require a local reference oscillation locked to the phase of the received signal. The demodulator is fairly simple in structure, and yet it demodulates complex angle modulated signals in which the phase variation due to any one bit is spread over a period longer than one bit interval in order to reduce spectrum occupation. To do this, the demodulator does not detect data directly from the received signal itself, but from a window extending over P symbols and based on a signal related to the received signal and comprising elementary pulses which are amplitude modulated and phase shifted relative to one another. The sequence of binary digits is then established on the basis of a demodulation algorithm performed on samples taken at the bit rate from the autocorrelation signal.